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Old 6th January 2012, 12:50 PM   #1271
Mark Kravchenko --- www.kravchenko-audio.com
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As a fly on the wall....

Thanks Art for your time and carefully made measurements. They display a great mind at work.

Ans as for David improving Hornesp. I think that's a given. Never have I seen such a commitment to producing so accurate a simulator. All who use it are indebted.

And few have taken the time to verify it's results. The more carefull measurements shared the better.
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Old 6th January 2012, 03:49 PM   #1272
tb46 is offline tb46  United States
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Default Multiple Speaker SPL

Hi David,

I hope that in evaluating the multiple speaker SPL output level you will not loose the radiator size and frequency dependent relationship of the acoustic radiation resistance.

Regards,
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Old 6th January 2012, 05:59 PM   #1273
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Originally Posted by David McBean View Post
Hi Art,

I tried simulating two parallel speakers in AkAbak - the predicted system response is consistent with your test measurement results (+6dB across the entire frequency range).

It would seem there is something in the established theory that I am missing :-).

I can replicate your results by a simple change to one line of the Hornresp code, but I need to understand what is going on first (not sure how long it will take me). To cover all situations perhaps I should include a "coherent / non-coherent" option in the Multiple Speakers tool.

Thanks again for highlighting this issue - hopefully Hornresp will eventually be a better product as a result of your very valuable feedback.

Kind regards,

David
Interesting to hear the difference between Akaback and Hornresp, seems in most respects they usually give similar results.

Glad to hear a simple change to one line of the Hornresp code may be a “fix”.

In post #1263 you wrote “In theory, for two speakers connected in parallel compared to one speaker, the difference at high frequencies should be 3 dB and the difference at low frequencies should be 6 dB”.

About the same time, on the full range forum, in the thread ” high efficiency speakers - how much power do they really need?” several people had been observing only a 3 dB gain indoors when doubling speakers.

If it would not have been for both observations disagreeing with my past experience outdoors, I may not have bothered doing the outdoor test with the two way horn speakers that highlighted the inconsistency between Hornresp and measured results (and Akaback results).

If there are any other tests you would like to see done to help in confirmation of any changes you decide to implement in Hornresp, don’t hesitate to PM me.

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Old 6th January 2012, 07:06 PM   #1274
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My outdoor measurements show a pretty uniform 6dB increase as well - when voltage remains constant and drivers are connected in parallel. Of course, if power is made constant, then drive voltage must be reduced as (parallel connected drivers) impedance is reduced. This brings the decibel increase back to 3dB. Also, I do see a smidge of a falloff at higher frequencies, where sound sources are acoustically distant.

Some sample datasets can be seen here:Click on the 2006 event, because that year we published charts of some speakers in singles, duals and even quads in some cases. The direct radiators were given a slight advantage, in that we ran them as duals or quads when comparing them to single basshorns. You can look at an individual subwoofer system and see the 2v/10M SPL chart of a single cabinet and compare it with the measurement made using two subs. The charts are 6dB different all the way, until you get up past 100Hz, where the difference falls to maybe 4dB or 5dB. By this point, the basshorns are acoustically large and becoming directional but they are also near mass-rolloff. Summing up high appears to fall off just a smidge, but not by 3dB.

In the 2005 and 2007 events, we focused more on measuring each cabinet at multiple power levels, getting both SPL and distortion charts for each one. So there is useful information there, but we did not have time to do multiple cabinets those years. In 2006, we measured cabinets indidually and in groups, but we did not have time to run them at increasingly higher power levels. So you get a little bit different information each year.
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Old 6th January 2012, 07:57 PM   #1275
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Originally Posted by Wayne Parham View Post
My outdoor measurements show a pretty uniform 6dB increase as well -
-snip-
The charts are 6dB different all the way, until you get up past 100Hz, where the difference falls to maybe 4dB or 5dB. By this point, the basshorns are acoustically large and becoming directional but they are also near mass-rolloff. Summing up high appears to fall off just a smidge, but not by 3dB.
Wayne,

I would bet that if your bass horns were angled in slightly to point exactly at the mic (as in my recent test of the small woofer/tweeter horn cabinet) that "smidge" lower summing up high would also become a full 6 dB increase.

It appears that the 12Pi's 30 Hz response increases by about 1.5 dB over the 6 dB overall response increase going from one to two, how does that compare to the Hornresp LF prediction?

If you have tested four 12Pi's together, that would also be another interesting comparison of measured vs. Hornresp.

Even though to many, a 1.5 dB difference does not amount to much, the fact that your FLH do show a LF increase, and both types of my quite different TH show no LF increase in pairs should be of further interest to David McBean.


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Old 7th January 2012, 12:07 AM   #1276
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I have a hard time figuring out the logic of your calculations, the only flaw I can see is that they disagree wth actual testing, which is done in dB SPL.
Hi Art,

I found the error in my calculations - I was summing the sound powers rather than the pressures :-). Not sure how I came to do this as I got it right in the Combined Response tool, which operates quite similarly in many respects (apart from having a phase difference between the two signals being combined).

The calculations should have been as follows:

Sound intensity from one speaker I = 1 / (4 * Pi * 2 ^ 2)
Sound pressure from one speaker P = (I * 344* 1.205) ^ 0.5

SINGLE SPEAKER:

SPL = 20 * Log10(1 * P / 0.00002) = 103.14 dB

TWO SPEAKERS:

SPL = 20 * Log10(2 * P / 0.00002) = 109.16 dB

The difference is now +6.02 dB - exactly as it should be :-).

I will post a message on the Hornresp thread when the error has been corrected and an update is released.

Thanks again for all your help.

Kind regards,

David
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Last edited by David McBean; 7th January 2012 at 12:12 AM.
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Old 7th January 2012, 12:19 AM   #1277
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Quote:
Originally Posted by weltersys View Post
I would bet that if your bass horns were angled in slightly to point exactly at the mic (as in my recent test of the small woofer/tweeter horn cabinet) that "smidge" lower summing up high would also become a full 6 dB increase.
Hmm, I dunno. The measurement distance was 10 meters (32.8 feet or 393 inches) and the center-to-center distance between subs standing side-by-side is around two feet, even the largest models. That means each was about 2° off-axis. I'm pretty sure the difference between 0° and 2° is negligible, not even measurable.
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Old 7th January 2012, 12:32 AM   #1278
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Originally Posted by Wayne Parham View Post
Hmm, I dunno. The measurement distance was 10 meters (32.8 feet or 393 inches) and the center-to-center distance between subs standing side-by-side is around two feet, even the largest models. That means each was about 2° off-axis. I'm pretty sure the difference between 0° and 2° is negligible, not even measurable.
One of those things that can't actually be resolved without measurement, so I won't argue the point.

I still am curious about this:

It appears that the 12Pi's 30 Hz measured response increases by about 1.5 dB over the 6 dB overall response increase going from one to two, how does that compare to your Hornresp LF prediction for two cabinets in half space?
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Old 7th January 2012, 12:34 AM   #1279
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Originally Posted by David McBean View Post
Hi Art,

I found the error in my calculations - I was summing the sound powers rather than the pressures :-). Not sure how I came to do this as I got it right in the Combined Response tool, which operates quite similarly in many respects (apart from having a phase difference between the two signals being combined).

The difference is now +6.02 dB - exactly as it should be :-).

I will post a message on the Hornresp thread when the error has been corrected and an update is released.

Thanks again for all your help.

Kind regards,

David
Cool, will this have an effect on the simulated LF response of multiple TH ?
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Old 7th January 2012, 12:58 AM   #1280
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I hope that in evaluating the multiple speaker SPL output level you will not loose the radiator size and frequency dependent relationship of the acoustic radiation resistance.
Hi Oliver,

That is the one thing that still puzzles me about Art's test results - and now those also posted by Wayne for dual cabinets.

I would have expected to see an additional 3 dB gain at low frequencies due to the mutual interaction between the two closely-coupled radiating horn mouths - which serve to increase the radiation resistance and therefore the acoustical output power.

I understand why it is not shown in my AkAbak simulations - the program is assuming that the two speakers are separated, not closely coupled.

At this stage I am not exactly sure what to do :-).

Kind regards,

David
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